Asymmetrical ballast transformer

ABSTRACT

A ballast transformer and system using the ballast transformer to couple power to a plasma load. The ballast transformer has a magnetic core, a first primary winding on a primary side of the magnetic core, a secondary winding on a secondary side of the magnetic core, and a second primary winding connected in series with the first primary winding and wound in proximity to the secondary winding on the secondary side of the magnetic core. The first primary winding is connectable to the AC power source, and the secondary winding is connectable to the plasma load via a coaxial cable.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 17/318,852, filedMay 12, 2021, which is a continuation of U.S. Ser. No. 17/085,604, filedOct. 30, 2020 (now U.S. Pat. No. 11,019,713), which claims priority toPCT/US/20/28373 (entire contents of which are incorporated herein byreference), filed Apr. 15, 2020 entitled “Asymmetrical BallastTransformer,” which is related to and claims priority to U.S. Ser. No.62/834,119 filed Apr. 15, 2019, entitled “Asymmetrical BallastTransformer,” the entire contents of which are incorporated herein byreference. This application is related to and claims priority to U.S.Ser. No. 62/834,947 filed Apr. 16, 2019, entitled “Waveform Detection ofStates and Faults in Plasma Inverters,” the entire contents of which areincorporated herein by reference. This application is related to andclaims priority to U.S. Ser. No. 62/834,545 filed Apr. 16, 2019,entitled “Frequency Chirp Resonant Optimal Ignition Method,” the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of Invention

The invention relates to the use of transformers for power coupling toreactive loads, such as plasmas, and other loads where transients needto be suppressed.

Discussion of the Background

Plasmas have been used extensively in a wide variety of industrial andhigh technology applications including, for example, semiconductorfabrication, various surface modifications, and coatings of reflectivefilms for window panels and compact disks. Plasmas ranging in pressurefrom high vacuum (<0.1 mTorr) to several Torr are common and have beenused for film deposition, reactive ion etching, sputtering and variousother forms of surface modifications. For example, gas plasmas are knownfor the treatment of plastics and molded substrates (e.g., thermoplasticolefin substrates used as bumpers and fascia in the automotive industry)to improve adhesion of subsequently applied coating layers. Themodification typically is a few molecular layers deep, thus bulkproperties of the polymeric substrate are unaffected. A primaryadvantage of using plasma for such purposes is that it results in an“all dry” process that generates little or no effluent, does not requirehazardous conditions such as high pressures, and is applicable to avariety of vacuum-compatible materials, including, inter alia, silicon,metals, glass and ceramics.

It is commonly known to use plasma, typically O₂ plasmas, as a means ofremoving hydrocarbon and other organic surface contaminants from varioussubstrates. However, because of the short lifetime of these reactantsand their line-of-sight reactivity on the surface, these highlyactivated reactants are not especially well-suited for surface cleaningof irregular surfaces, unpolished or roughened metallic surfaces, orsurfaces having a three-dimensional topography.

Also, use of plasma at reduced pressures has several disadvantages inthat the substrate to be treated or cleaned must be placed under vacuumand must be capable of surviving under such reduced pressure conditions.Use of a plasma at or above atmospheric pressure avoids these drawbacks.

Yet, the coupling of power into atmospheric pressure plasmas is notstraight forward, especially during the time frame when the gastransitions into a plasma. The gas presents a high impedance to thepower source, while the resultant plasma appears as a low impedance loadto the power source, with the transition from these states resulting ina dynamic change in impedance and current surges.

SUMMARY

In one embodiment of the invention, there is provided a ballasttransformer having a magnetic core, a first primary winding on a primaryside of the magnetic core, a secondary winding on a secondary side ofthe magnetic core, and a second primary winding connected in series withthe first primary winding and wound in proximity to the secondarywinding on the secondary side of the magnetic core.

In one embodiment of the invention, there is provided a system forcoupling power to a plasma load, the system comprising: an alternatingcurrent (AC) power source; a ballast transformer having a magnetic core,a first primary winding on a primary side of the magnetic core andconnected to the AC power source, a secondary winding on a secondaryside of the magnetic core, and a second primary winding connected inseries with the first primary winding and wound coaxial to the secondarywinding on the secondary side of the magnetic core. The system has acoaxial cable for connecting the secondary winding to the plasma load.

In one embodiment of the invention, there is provided a method forproviding/coupling power to a plasma load, the method comprising:providing power from an AC power source to a plasma load via theasymmetric ballast transformer in any of the statements above having asufficient leakage inductance to prevent current surges; and ignitingand developing a full atmospheric pressure plasma.

In one embodiment of the invention, there is provided a method forcoupling power to a plasma load using the system described above, themethod includes coupling power from the AC power source to the plasmaload via an asymmetric ballast transformer having a leakage inductanceand attached to a coaxial cable with capacitance; while in a no-plasmastate, generating a near-resonance voltage on the secondary side due tothe leakage inductance and the capacitance; and igniting a plasma at thenear-resonance voltage and thereafter decreasing an operationalfrequency of the AC power source.

It is to be understood that both the foregoing general description ofthe invention and the following detailed description are exemplary, butare not restrictive of the invention.

BRIEF DESCRIPTION OF THE FIGURES

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1A is a schematic of an atmospheric plasma source;

FIG. 1B is a schematic of a system for coupling power to an atmosphericplasma;

FIG. 2 is a schematic circuit model of a ballast transformer coupled toa variable resistance load;

FIG. 3 is a schematic of an asymmetrical ballast transformer of thepresent invention;

FIG. 4 is a schematic graph of frequency vs impedance sweep of using aballast transformer for different loads;

FIG. 5 is a schematic graph of a current transfer frequency sweep fordifferent loads coupled to a ballast transformer, one with no load(pre-ignition of plasma) and the other at post-ignition;

FIG. 6 is a schematic graph of a current transfer frequency sweep for apost ignition plasma load and a fully developed plasma load;

FIG. 7 is a flow chart detailing a method of the present invention forcoupling an alternating current (AC) voltage source to a load.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A is a lengthwise cross-sectional view of an example of an APplasma source 102. The AP plasma source 102 includes an axiallyelongated plasma-generating chamber 104 or other structure that servesas a ground electrode for generating a plasma and that serves as aconduit for flowing gases into the plasma. The plasma-generating chamber104 may be enclosed in an electrically- and thermally-insulating housing(not shown). A “hot” or powered electrode 106 is located in theplasma-generating chamber 104. Electrical connections to the hotelectrode 106 may be made through a dielectric structure 108 located atthe proximal end of or in the plasma-generating chamber 104. One or moregas inlets 110 may be formed through the dielectric structure 108 influid communication with the plasma-generating chamber 104. The gasinlets 108 may be placed in fluid communication with a gas supplysource. Accordingly, the gas inlets 110 provide a flow path forplasma-generating gas fed to plasma-generating region 112 within theplasma-generating chamber 104 proximate to the hot electrode 106. Inoperation, the plasma is generated in region 112 and subsequently flows(with the gas flow) toward a nozzle 114 positioned at a distal end ofthe plasma-generating chamber 104.

Generally, operating parameters associated with the AP plasma source 102are selected so as to produce a stable plasma discharge. Control 116having a processor is used for setting and controlling the operatingparameters which depend on the particular application ranging, forexample, from nanoscale etching of micro-fabricated structures ordevices (e.g., MEMS devices) to removing large areas of paint fromaircraft carriers. Examples of operating parameters are provided belowwith the understanding that the teachings herein are not limited by suchexamples. In the case of generating an air plasma, the rate at which theair is fed to the AP plasma source 102 may range from 1×10⁻⁶ SCCM to1×10⁶ SCCM. The feed pressure into the AP plasma source 102 may rangefrom 1 Pa to 1×10⁷ Pa. The power level of the electrical field drivingthe plasma may range from 1×10⁻⁶ W to 1×10⁶ W. The drive frequency ofthe electrical field may range from DC (0 GHz) to 100 GHz. Theseparation distance, i.e. the distance from the nozzle exit to theexposed surface of the material to be removed, may range from 1×10⁻⁶ mto 40 cm. The scan speed, i.e. the speed at which the AP plasma source102 may be moved across (over) the surface of the material, may rangefrom 1×10⁻⁴ m/s to 10 m/s. Related to the scan speed and power is thetime averaged power density. Also related to the scan speed is the dwelltime. i.e., the period of time during which a particular area of thematerial is exposed to the plasma plume, which may range from 1×10⁻⁹ sto 1×10³ s.

In one embodiment of the present invention, AP plasma source 102 has aconverging nozzle (i.e., a straight conical cross-sectional flow areawithout being followed by a diverging section), has been fabricated andevaluated. The AP plasma source repeatably and reliably produces aplasma plume which may include the production of shock waves. The APplasma source generates an air plasma using air at about roomtemperature as the feed gas. The air may be fed to an AP plasma sourceof this type at a pressure ranging from 30-110 psi and at a flow rateranging from 1-7.5 CFM. In another example, the pressure range is 65-95psi. In another example, the flow rate range is 1-4 CFM. Pressureshigher than 110 psi may also be implemented to produce shock waves. In amore general example, the pressure may be 30 psi or greater and the flowrate may be 1 CFM or greater.

Under these conditions, at plasma ignition, there is a (typically small)arc from the driven or “electrically hot” electrode to the chamber wall,and the gas flow “expands” the spatially confined arc into a diffusedvolume of plasma or plasma plume 118 extending out of the outlet 114.The electrical impedance before and after plasma ignition and during theexpansion of the arc vary greatly as detailed below.

The present invention provides as shown in FIG. 1B a system forproviding power to the plasma during these changing load resistanceconditions by way of an inverter 120 (controlling for example the ACfrequency of a square wave pulse signal) and a ballast transformer 122.In this system, a leakage inductance of the ballast transformer 122serves the purposes of both a) limiting the current into a variable loadwhen driven by a fixed voltage AC source and b) providing a resonancewith cable capacitance and therefore can provide a high voltage toignite a plasma.

FIG. 2 is a schematic circuit model of asymmetric ballast transformer122 of the present invention for coupling to a variable load Rv such asa plasma load, including but not limited to an atmospheric pressureplasma pen (discussed above) or a cutting torch (discussed below). Asshown in FIG. 2 , an AC voltage source 130 is coupled to transformer 122by coupling connections 134. The AC voltage source can provide an ACwaveform which may be sinusoidal, square wave, or other arbitrary pulsedor bi-polar waveform, and provide a waveform whose frequency can bevaried. The voltage source 130 supplies current which flows through theprimary windings 1. The current through windings 1 induce current flowthrough the secondary windings 2 of transformer 102, producing a step upor step down AC voltage which appears across the variable plasma loadR_(v). A coaxial connection 140 is used in this circuit to connect thetransformer 122 to the variable plasma load R_(v), but other types ofelectrical interconnects with or without filtering could be used inaddition or instead of the coaxial connection 140. As shown in FIG. 2 ,a leakage inductance 138 and cable capacitance 142 (from the coaxialcable) appear in this circuit.

In general, ballast transformers have a leakage inductance that appearsin a simple analysis as a separate inductor (leakage inductor) in serieswith the primary and or the secondary. If the leakage inductance issufficiently large, the present inventors have realized (as noted above)that this leakage inductance will serve both to a) limit the currentinto a variable load when driven by a fixed voltage AC source and b)provide a resonance (with the cable capacitance) and therefore canprovide a high voltage to ignite a plasma.

Existing transformers with a two pole or three pole transformer corerequire either a larger core with lower magnetic path length to crosssectional area ratio and extra magnetic path extension in thetransformer core in order to reduce coupling to an acceptable valuewhere a transient load would not adversely affect a voltage source suchas voltage source 130. Alternatively, the transformer would need finerwire with more turns and thick bobbin walls for a coaxial design on atwo pole transformer core in order to suppress current surges. Both ofthese approaches are undesirable.

Accordingly, the present inventors have realized that, for aconventional two pole core design to suppress current surges, a set oflarge bobbins along with a fine wire size would be necessary. Indeed,because of the limited wire sizes that are practical, many turns wouldbe necessary to achieve a sufficient flux density. Yet, this approachcomes with excessive wire heating even for a 1-3 kW transformer forexample having a ˜50-100 mm (height and width) 2 pole transformer core,with a core area of each pole being ˜320 mm²-600 mm². Furthermore, thepresent inventors have realized that, if only a single primary windingwere placed on one pole of the core and only a secondary winding were onthe other pole, then it is impossible to obtain coupling as high as0.97.

Accordingly, using conventional measures, one either a) obtains atransformer with limited power rating or b) cannot obtain enoughcoupling. These deficiencies are especially problematic when thevariable load is a plasma, where the on state and the off state presenta tremendous change in impedance nearly instantly, which can result inexcessive current flow and damage to the power supply and power couplingequipment.

Asymmetric Ballast Transformer

In view of the problems noted above for the ignition and operation of anatmospheric pressure plasma, the present inventors have devised asolution utilizing both a two pole winding design with a coaxial windingof a second primary winding on the secondary side of a transformer, Thissolution provides an asymmetric ballast transformer permittingadjustment of the primary windings so that some of the primary windingsare on the primary pole and the rest of the primary windings aredisposed in a vicinity of and preferably coaxial with the high voltagesecondary coil on the second pole.

FIG. 3 is a schematic of a ballast transformer of the present invention.As shown in FIG. 3 , a magnetic flux circuit comprises a transformercore 300 forming a magnetic loop (which could include air gaps notshown) linking a primary side 302 of the transformer to a secondary side312 of the transformer. The primary side 302 comprises a first primarywinding 304 of wire W1 on bobbin 306. Wire W1 connects to an AC powersource (not shown in FIG. 3 ), but similar to voltage source 130 in FIG.2 . The secondary side 312 comprises a second winding 314 of wire W2 onbobbin 316. Wire W2 connects to a variable load not shown in FIG. 3 ,but similar to variable load R_(v) in FIG. 2 .

In FIG. 3 , bobbin 316 is illustrated for a high voltage secondary.Bobbin 316 has wire W1 wound around it. In one embodiment of theinvention, bobbin 316 fits inside primary bobbin 326 to provide couplingthereto. The position of primary bobbin 326 on the second pole can varyfrom design to design to provide an adjustable coupling to the secondarywinding 324 and/or to the transformer core 300. Primary bobbin 326typically has a lower coupling to the transformer core than either thesecondary winding 324 or the primary winding 304 on bobbins 316 and 306,respectively.

In one embodiment of the invention, the required number of turns for thetransformer's primary are distributed between the two primary bobbins306 and 326 in order to set the coupling for an appropriate leakageinductance, while the total number of windings on the primary bobbinsremains the same as if there were only one primary bobbin, thusobtaining appropriate excitation or magnetization inductance, andthereby controlling maximum flux while allowing larger wires on thebobbins than otherwise would be the case when the primary windings werecoaxial on only one pole. In one embodiment of the invention, bobbin 326is insulated although insulation may not be necessary if bobbin 326 isof a size to where it can reside at the bottom of the secondary windingwhere the voltage is lower than at the top side of the secondarywindings.

Accordingly, in one embodiment of the invention, the primary winding ontransformer core 300 is split by the presence of second primary winding324 in proximity to (e.g., wrapped around or coaxially surrounding) thesecondary winding 314. This second primary winding 324 (connected inseries with the winding primary winding 304) can be a non-coaxial and/ora coaxial winding relative to the secondary winding 314 so that it ispossible to control the coupling coefficient (leakage inductance) andoptimize the trade-off between maximum flux density, core heating, andwire losses without the necessity of auxiliary adjustable flux paths. Inone embodiment, the relative positions of bobbin 306, bobbin 316, and/orbobbin 326 to the transformer core (and/or to each other) can beadjusted or can otherwise be fixed at different relative positions.

In one embodiment of the invention, by keeping the number of turnsconstant, the exact coupling may be adjusted by moving turns fromprimary 1 (winding 304) to primary 2 (winding 324) or vice versa. Ineffect, turns can be moved back and forth between primary bobbins toadjust the coupling and leakage inductance. If more turns are on primary2 and less on primary 1, then the coupling is increased withoutaffecting the turns ratio or open circuit (no load) output voltage.Reversing the situation, more turns on primary 1 and less on primary 2decreases the coupling. Less coupling makes the leakage inductanceincrease while more coupling makes it decrease.

The numerical values given below are merely illustrative and notlimiting of a asymmetrical ballast transformer of the present invention.Typical values for operation of the ballast transformer of the presentinvention are 0-350 mTeslas, 0.97 coupling on primary, net loss <50 Wbetween 20-500 kHz, 1 kV-50 kV peak volts pre-ignition, 0.50-5 kV voltpeak operating, 0 volts output post-ignition state.

Below are details of a constructed asymmetrical ballast transformer ofthe present invention.

Ballast Transformer Design

-   -   Operational Input: Pulsed 300 V above ground signal at pulse        frequency from 20-500 kHz    -   Transformer Design:        -   Primary Rating: 230 VAC        -   Epoxy coating or other coating to hold primary wire and            secondary wire in place on bobbins and to prevent vibration            in use.        -   First Primary Winding: wire size #12 AWG, 1-15 turns        -   First Primary Winding Inductance: 0.5-10 μH at 10 KHz, no            core        -   Second Primary Winding: wire size #12 AWG, 1-15 turns        -   Second Primary Winding Inductance: 0.5-10 μH at 10 KHz, no            core        -   Total Primary Windings: 2-30 turns        -   Total Primary Winding Inductance: 500-2000 μH at 10 KHz,            with core, Q=300        -   Secondary Winding: wire size #22 AWG, ˜200 turns, layered            windings        -   Secondary Winding Inductance: 100 to 1000 μH at 10 kHz, no            core            -   50 to 5000 mH at 10 kHz, with core, Q=500        -   Measured Leakage Inductance: 5-100 μH at 10 kHz, with core,            Q=30

A further embodiment of the present invention is that bobbin 326 can bedisposed offset from the transformer core, that is that the primary wireW1 on bobbin 326 is moved away from the core (where the operating fluxand thus heating is the greatest). Flux near wire W1 is somewhat higherthan elsewhere in the core.

Further, in another embodiment, the bobbin may be perforated for airflow or liquid cooling tubes along the core inside of the winding. Insome cases, it is also advantageous to offset bobbin 326 on the core toallow more wire exposure into the window region 330 of the transformercore, and thereby move the magnetic flux distribution in the core toprevent localized saturation of the core. FIG. 3 shows bobbin 326 in oneoffset position, but it is also possible to have bobbin 326 centeredaround bobbin 316 and to only offset bobbin 306 on the primary side ofthe transformer.

Coil layers of the windings on the bobbins may also be separated forbetter cooling and less current crowding. Flux is not the same aroundthe core during any mode of operation since the circuit with capacitiveoutput causes significant circulating current so that the circulatingpower is typically 1.6 times the real output power. Such a relationshipis necessary for ballasting. The transformer core may be un-gapped formaximum power output, but in another embodiments a gapped core isutilized to minimize saturation. This design does not necessarily have,but could utilize, a center tapped primary although center tapping wouldreduce power handling and/or increase size.

In one embodiment of the invention, the primary bobbin 306 (as notedabove) is offset from primary side 302 of the transformer core. Thisoffset allows magnetic flux to leak out and be intercepted by secondprimary winding 324 wound on bobbin 326.

In one embodiment of the invention, one of the primary or secondarywindings provides tight coupling while the other provides loose couplingwhile simultaneously providing a) enough leakage inductance to limitflux density to a safe level, b) at least a turns ratio to develop theoperating or developed plasma voltage and more, and c) a reasonableleakage inductance for resonance condition for ignition and use thatsame leakage inductance for ballast when there is a developed plasma. Inone embodiment of the invention, the leakage is adjusted by constructionof the ballast transformer components so as not to change the turnsratio all the while keeping the transformer compact while avoiding extraferrite flux path elements.

In one embodiment of the invention, it is desirable to minimizeinterwinding capacitance. As shown in FIG. 3 , there is a two ‘pole’ferrite core but the primary is not wound over the top of the ferritecore and the secondary is not wound over the top of the ferrite core.This is to avoid electrostatic stray capacitive coupling between theprimary and secondary. Furthermore, in one embodiment of the invention,one part of the primary coaxial to the secondary is at the ‘grounded’ orlow voltage end. This arrangement further reduces the effect of straycapacitive coupling by grounding that end of the secondary winding. Atplasma ignition, the plasma impedance transitions very quickly betweenvery high (megaohms or greater) impedance to a near short circuit (lessthan an Ohm). This means the load voltage on the plasma electrode dropsrapidly (for example in a very few nanoseconds) which can generate alarge capacitively coupled transient into the primary which can damagethe drive devices in control 116 or inverter 120 like IGBTs or FETs. Inone embodiment of the invention, this effect is minimized by the opentop ferrite core design shown in FIG. 3 , reducing the interwindingcapacitance between the primary and secondary winding.

Benefits of the present invention also not having windings 314 and 324bet coaxial except for a small section on the cold end (the low voltageend) of secondary 312 so that high electric fields are not created atthe top of secondary 312 and therefore the possibility of a damagingcorona discharge in air is reduced and the interwinding capacitance isreduced. Moreover, the magnetic flux is not uniform in the core givenits operation during the three operational states (ignition of arc,expansion of arc, plasma maintenance). Since the core would begin tosaturate first under primary 1 (winding 304) but not under primary 2(winding 324) where the magnetic flux is much less, core saturationevents that could damage the driving circuit control 116 (due to highcurrents in turn due to loss of inductance) can be detected, and theinverter 120 drive shut down before damage occurs.

One optional embodiment of the inventive asymmetric ballast transformerdesign is that the bobbins, if needed, can be stacked end-to-end againsteach other using their end flanges so that no spacer is required.

Another optional embodiment of the inventive asymmetric ballasttransformer design is that the primary is not necessarily disposed overthe high voltage end of the secondary, thereby avoiding most of thecapacitive coupling which is one significant way that ignitiontransients can be transferred from the spark gap in the torch/pen to theprimary and to the transistor bridge devices (control 116 and inverter120) in the voltage source. This disposal of the primary not over thehigh voltage end of the secondary may also improve cooling to thesecondary side of the transformer compared to a coaxial design whichwould require more turns and finer wire, and thus be limited to loweroperational powers.

Typically, for the asymmetrical ballast transformer of the presentinvention, a coupling coefficient is about 0.97, and a magnetizationinductance (inductance of the primary winding 304 and secondary winding314) is about 30 times greater than the leakage inductance. According toone embodiment of the invention, the leakage inductance is preferably ofa value that limits current in the primary side at the instant theplasma ignites. Plasma ignition represents a tremendous change inimpedance from that of an open circuit prior to ignition to thatappearing almost as a short circuit after plasma ignition.

FIG. 4 is a graph of frequency vs impedance sweep characteristic of theballast transformer circuit for a “no plasma” case and for a developedplasma case. Two points/frequencies marked are for plasma operation (tothe left, 90.27 kHz) and pre-ignition (to the right, 149.6 kHz). Thesefrequencies will vary with a particular coax cable type and length (FIG.2 , coaxial cable 140 and its cable capacitance 142). If the cablecapacitance changes, the operating points/frequencies change for thesame output. Note the scale depicted is magnitude dB relative to 1 voltat the output of the cable (i.e. at the pen/torch/plasma port). 80 dBvolt is 10,000 volts. 68 dB volt is 2,500 volts amplitude or peak. InFIG. 4 , the no plasma curve represents a very high load impedance (herecalculated for 100 k Ohms, but it may be 1 megaohm or higher). In FIG. 4, the fully developed plasma curve represents a load of 2000 Ohms. Thetype of plasma pen/torch used for these calculations was assumed to be ahigh voltage plasma type with a relatively high impedance while running.

In one embodiment of the invention, the frequency of operation can bemoved from 149.6 kHz toward a lower frequency (toward the peak resonancefrequency) in order to develop higher ignition voltages (than wouldexist at 149.6 kHz) and thereafter moved to even lower frequencies (onceignited) to couple more plasma power once ignited and developed.

One plasma condition that is not shown in FIG. 4 is the stateimmediately post ignition when the plasma is spatially confined into aconductive filament small in size and has an impedance of 1 Ohm or less.This condition is depicted in FIG. 5 , a graph comparing input currentfor pre-ignition and post-ignition. In FIG. 5 , the scale on the rightis in dB Amperes relative to 1 Ampere peak. Specifically, FIG. 5 showsthe input current as a function of frequency for a no load pre-ignition,and shows the input current as a function of frequency for a 1 Ohmpost-ignition state (not a fully developed plasma). At startup, theinput current with no load condition (simulation is 100 k Ohms) is atabout 32 dB or 39.8 Amperes peak. When the plasma ignites, the inputcurrent is relatively small and has a relatively low impedance ascompared to the fully developed plasma state and simulated here with aresistance 1 Ohm. Note that the input current at plasma ignitionactually drops to about 21 dB or 11.2 Amperes peak. Thereafter, as shownin FIG. 6 , the plasma develops and the input current increases as thefrequency of operation is lowered to 90.27 kHz or lower.

More specifically, as the plasma develops, the impedance increasesmoving the current from the post-ignition current curve to theplasma-run current curve, and the frequency is adjusted to 90.27 kHz inthis example to develop full power. Thus the ballast transformer is usedto permit the system to generate ignition voltages (FIG. 4 ), withstandthe sudden load transition from very high to near shorted conditions(FIG. 5 ) and then smoothly move to full power plasma (FIG. 6 ).

In general, a number of advantages are provided by inventive asymmetricballast transformer of the present invention including but not limitedto:

-   -   one can separately specify the number of primary turns and thus        the excitation flux of the transformer to control heating,        possible core saturation, and the coupling coefficient;    -   the lower capacitive coupling between primary and secondary        compared to a single pole coaxial winding deign results in lower        transient transfer from the secondary side to the primary side,        while nevertheless obtaining the coupling benefits from a        coaxial winding at the bottom of the HV bobbin; and    -   the secondary windings need not be covered (or vice versa the        primary winding is not covered) for the most part, and thus can        be cooled more effectively.

The ballast transformer of this invention may work with any turns ratioincluding for example those less than 1 which could be used to drive lowimpedance cutting torches. Ignition with these work tools is a problemtypically requiring contact and withdrawal of the electrode. It would bepreferable if its power coupling could also be current-limited orballasted in some way. While the cable capacitance in conventionalcutting torches is almost non-existent (because the transformer isplaced closed to the working electrode), in the present invention, alarger than conventional cable capacitance (or a supplemental capacitorto ground) could be used in order for enough voltage to develop under ano load condition to ignite the torch. In this case, add on igniters asconventionally used may be avoided. For example, U.S. Pat. No. 7,022,935(the entire contents of which are incorporated herein by reference)describes a plasma cutting torch having an output electrode and a plasmacutter starting circuit configured to generate a pilot arc at the outputelectrode.

Hence, the “pen/torch” reference in FIG. 1B and the variable plasmaresistance of FIG. 2 refer to the present invention's utility in thesetypes of cutting torches typically having low impedances.

Computer Control

It will be understood that the control 116 schematically illustrated inFIGS. 1A and 1B may also be representative of one or more types of userdevices, such as user input devices (e.g., keypad, touch screen, mouse,and the like), user output devices (e.g., display screen, printer,visual indicators or alerts, audible indicators or alerts, and thelike). Control 116 may have a graphical user interface (GUI) controlledby software for display by an output device, and one or more devices forloading media readable by the controller 212 (e.g., logic instructionsembodied in software, data, and the like). The control 116 may includean operating system (e.g., Microsoft Windows® software) for controllingand managing various functions thereof.

FIG. 7 is a flowchart detailing a method of the present invention forpowering a plasma load.

In step 701, coupling power from an AC source to a variable plasma loadvia an asymmetric ballast transformer having a leakage inductance and acoaxial capacitance to ground. The variable plasma load comprising forexample the atmospheric pressure plasma source or the low impedancecutting torches discussed above.

In step 703, while in a no-plasma state, generating a nearresonance-voltage on the secondary side due to the leakage inductanceand the capacitance.

In step 705, ignite a plasma at the near-resonance-voltage, andthereafter lower the operational frequency driving the plasma. In thisstep, the fully developed plasma load is resistive with the leakageinductance acting as a low pass filter preventing high frequencytransients from propagating backwards into the AC source.

Viewed differently, these steps in general provide power from an ACpower source to a plasma load via an asymmetric ballast transformerhaving a sufficient leakage inductance to prevent current surges; andignite and develop a full atmospheric pressure plasma.

It will be understood that one or more of the processes, sub-processes,and process steps described herein may be performed by hardware,firmware, software, or a combination of two or more of the foregoing, onone or more electronic or digitally-controlled devices for exampleadjusting the variable capacitors and/or the relative bobbin positionsand/or the power level of the AC source 130. The software may reside ina software memory (not shown) in a suitable electronic processingcomponent or system such as, for example, the control 116 schematicallydepicted in FIG. 1A. The software memory may include an ordered listingof executable instructions for implementing logical functions (that is,“logic” that may be implemented in digital form such as digitalcircuitry or source code, or in analog form such as an analog sourcesuch as an analog electrical, sound, or video signal). The instructionsmay be executed within a processing module, which includes, for example,one or more microprocessors, general purpose processors, combinations ofprocessors, digital signal processors (DSPs), or application specificintegrated circuits (ASICs). Further, the schematic diagrams describe alogical division of functions having physical (hardware and/or software)implementations that are not limited by architecture or the physicallayout of the functions. The examples of systems described herein may beimplemented in a variety of configurations and operate ashardware/software components in a single hardware/software unit, or inseparate hardware/software units.

The executable instructions may be implemented as a computer programproduct having instructions stored therein which, when executed by aprocessing module of an electronic system (e.g., the control 116schematically depicted in FIG. 1A), direct the electronic system tocarry out the instructions. The computer program product may beselectively embodied in any non-transitory computer-readable storagemedium for use by or in connection with an instruction execution system,apparatus, or device, such as a electronic computer-based system,processor-containing system, or other system that may selectively fetchthe instructions from the instruction execution system, apparatus, ordevice and execute the instructions. In the context of this disclosure,a computer-readable storage medium is any non-transitory means that maystore the program for use by or in connection with the instructionexecution system, apparatus, or device. The non-transitorycomputer-readable storage medium may selectively be, for example, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, or device. A non-exhaustive list ofmore specific examples of non-transitory computer readable mediainclude: an electrical connection having one or more wires (electronic);a portable computer diskette (magnetic); a random access memory(electronic); a read-only memory (electronic); an erasable programmableread only memory such as, for example, flash memory (electronic); acompact disc memory such as, for example, CD-ROM, CD-R, CD-RW (optical);and digital versatile disc memory, i.e., DVD (optical). Note that thenon-transitory computer-readable storage medium may even be paper oranother suitable medium upon which the program is printed, as theprogram can be electronically captured via, for instance, opticalscanning of the paper or other medium, then compiled, interpreted, orotherwise processed in a suitable manner if necessary, and then storedin a computer memory or machine memory.

It will also be understood that the term “in signal communication” asused herein means that two or more systems, devices, components,modules, or sub-modules are capable of communicating with each other viasignals that travel over some type of signal path. The signals may becommunication, power, data, or energy signals, which may communicateinformation, power, or energy from a first system, device, component,module, or sub-module to a second system, device, component, module, orsub-module along a signal path between the first and second system,device, component, module, or sub-module. The signal paths may includephysical, electrical, magnetic, electromagnetic, electrochemical,optical, wired, or wireless connections. The signal paths may alsoinclude additional systems, devices, components, modules, or sub-modulesbetween the first and second system, device, component, module, orsub-module.

More generally, terms such as “communicate” and “in . . . communicationwith” (for example, a first component “communicates with” or “is incommunication with” a second component) are used herein to indicate astructural, functional, mechanical, electrical, signal, optical,magnetic, electromagnetic, ionic or fluidic relationship between two ormore components or elements. As such, the fact that one component issaid to communicate with a second component is not intended to excludethe possibility that additional components may be present between,and/or operatively associated or engaged with, the first and secondcomponents.

Exemplary Statements of the Invention

The following numbered statements of the invention set forth a number ofinventive aspects of the present invention:

Statement 1. A ballast transformer having a magnetic core (optionallyincluding gaps), a first primary winding on a primary side of themagnetic core, a secondary winding on a secondary side of the magneticcore, and a second primary winding connected in series with the firstprimary winding and wound coaxial to the secondary winding on thesecondary side of the magnetic core.

Statement 2. The ballast transformer of statement 1, having a resonanceassociated with a capacitance of a cable connected to the secondarywinding and a leakage inductance of the transformer.

Statement 3. The ballast transformer of statement 2, wherein theresonance may be associated with a capacitance of a capacitor connectedto ground from a power lead connected to the secondary winding.

Statement 4. The ballast transformer of statement 2, wherein theresonance comprises a high Q circuit when no load is present.

Statement 5. The ballast transformer of statement 2, wherein the leakageinductance opposes current surges at plasma ignition.

Statement 6. The ballast transformer of statement 1, wherein the ballasttransformer comprises a step-up transformer or a step-down transformer.

Statement 7. The ballast transformer of statement 1, wherein the secondprimary winding is displaceable from the secondary winding to alter acoupling coefficient of the transformer.

Statement 8. The ballast transformer of statement 1, wherein the secondprimary winding is displaceable from the secondary winding to alter aresonance frequency of the resonant transformer.

Statement 9. The ballast transformer of statement 1, wherein the secondprimary winding wraps around or coaxially surrounds the secondarywinding.

Statement 10. The ballast transformer of statement 1, wherein the secondprimary winding is offset axially from and surrounds the secondarywinding.

Statement 11. The ballast transformer of statement 1, wherein the secondprimary winding is displaceable from the secondary winding to alter acoupling coefficient of the transformer.

Statement 12. The ballast transformer of statement 1, wherein the secondprimary winding is displaceable from the secondary winding to alter aresonance frequency of the resonant transformer.

Statement 13. The ballast transformer of statement 1, further comprisingrespective bobbins for holding the first primary winding, the secondarywinding, and the second primary windings in place around the magneticcore.

Statement 14. The ballast transformer of statement 1, wherein therespective bobbins have holes for air cooling.

Statement 15. The ballast transformer of statement 1, wherein at leastone of the first primary winding and the secondary winding is disposedoffset from the magnetic core or the first primary winding is offsetaxially from the magnetic core.

Statement 16. A system for coupling power to a plasma load, the systemcomprising:

-   -   an alternating current (AC) power source;    -   the ballast transformer of any one or more of statements 1-15;    -   wherein    -   the first primary winding of the ballast is connectable to the        AC power source, and    -   the secondary winding of the ballast is connectable to the        plasma load by a coaxial cable.

Statement 17. The system of statement 16, wherein the ballasttransformer comprises a resonant transformer having a resonanceassociated with a) a capacitance of the coaxial cable connected to thesecondary wining and b) a leakage inductance of the ballast transformer.

Statement 18. The system of statement 16, wherein the ballasttransformer comprises a step-up transformer or a step-down transformer.

Statement 19. The system of statement 16, wherein the ballasttransformer comprises a resonant transformer having resonance associatedwith a) a capacitance of the coaxial cable and b) a leakage inductanceof the transformer.

Statement 20. The system of statement 16, further comprising aplasma-generating region connected in series with the secondary windingvia the coaxial cable.

Statement 21. The system of statement 20, wherein the ballasttransformer comprises a high Q circuit when no plasma exists in theplasma generating region and comprises a non-resonant resistive circuitwhen a plasma exists in the plasma generating region.

Statement 22. The system of statement 21, wherein a leakage inductanceof the ballast transformer opposes current surges at plasma ignition.

Statement 23. The system of statement 22, wherein the second primarywinding is displaceable from the secondary winding to alter a couplingcoefficient of the transformer.

Statement 24. The system of statement 22, wherein the second primarywinding is displaceable from the secondary winding to alter a resonancefrequency of the resonant transformer before plasma ignition.

Statement 25. The system of statement 16, wherein the variable plasmaload comprises an atmospheric pressure plasma or a cutting torch and/orthe variable plasma load plasma comprises a non-thermal plasma used forthe removal of organic contaminants, coatings, adhesives and sealantsfrom surfaces.

Statement 26. The system of statement 16, wherein the second primarywinding wraps around or coaxially surrounds the secondary winding.

Statement 27. The system of statement 16, wherein the second primarywinding is offset axially from and surrounds the secondary winding.

Statement 28. The system of statement 16, wherein the second primarywinding is displaceable from the secondary winding to alter a couplingcoefficient of the transformer.

Statement 29. The system of statement 16, wherein the second primarywinding is displaceable from the secondary winding to alter a resonancefrequency of the resonant transformer.

Statement 30. The system of statement 16, further comprising respectivebobbins for holding the first primary winding, the secondary winding,and the second primary windings in place around the magnetic core.

Statement 31. The system of statement 16, wherein the respective bobbinshave holes for air cooling.

Statement 32. The system of statement 16, wherein at least one of thefirst primary winding and the secondary winding is disposed offset fromthe magnetic core.

Statement 33. The system of statement 1, further comprising a controllerconfigured to at least one of:

-   -   control a gas flow through the plasma load, and    -   control an operational frequency of the AC power source.

Statement 34. The system of statement 33, wherein the controller isconfigured to:

-   -   via the ballast transformer in statement 1 comprising an        asymmetric ballast transformer having a sufficient leakage        inductance to prevent current surges, control the AC power        source such that a plasma is ignited and developed into a steady        state atmospheric pressure plasma.

Statement 35. A method using any of the system statements above forproviding/coupling power to a plasma load, the method comprising:

-   -   coupling power from the AC power source to the plasma load via        an asymmetric ballast transformer having a leakage inductance        and attached to a coaxial cable with capacitance;    -   while in a no-plasma state, generating a near-resonance voltage        on the secondary side due to the leakage inductance and the        capacitance; and    -   igniting a plasma at the near-resonance voltage and thereafter        decreasing an operational frequency of the AC power source.

Statement 35. A method using any of the system statements above forproviding/coupling power to a plasma load, the method comprising:

-   -   providing power from an AC power source to a plasma load via the        asymmetric ballast transformer in any of the statements above        having a sufficient leakage inductance to prevent current        surges; and    -   igniting and developing a steady state atmospheric pressure        plasma.

Numerous modifications and variations of the invention are possible inlight of the above teachings. It is therefore to be understood thatwithin the scope of the appended claims, the invention may be practicedotherwise than as specifically described herein.

1. A system for coupling power to a plasma load, comprising: analternating current (AC) power source; a ballast transformer having amagnetic core, a first primary winding on a primary side of the magneticcore and connected to the AC power source, a secondary winding on asecondary side of the magnetic core, and a second primary windingconnected in series with the first primary winding and wound coaxial tothe secondary winding on the secondary side of the magnetic core; and acoaxial cable for connecting the secondary winding to the plasma load,wherein the ballast transformer comprises a resonant transformer havinga resonance associated with a) a capacitance of the coaxial cableconnected to the secondary wining and b) a leakage inductance of theballast transformer.
 2. The system of claim 1, wherein the ballasttransformer comprises a step-up transformer or a step-down transformer.3. The system of claim 2, further comprising a plasma-generating regionconnected in series with the secondary winding via the coaxial cable. 4.The system of claim 3, wherein the ballast transformer comprises a highQ circuit when no plasma exists in the plasma generating region andcomprises a non-resonating, resistive circuit when a plasma exists inthe plasma generating region.
 5. The system of claim 4, wherein aleakage inductance of the ballast transformer opposes current surgeswhen the plasma is ignited in the plasma generating region.
 6. Thesystem of claim 5, wherein the second primary winding is displaceablefrom the secondary winding to alter a coupling coefficient of theballast transformer.
 7. The system of claim 1, wherein the plasmacomprises an atmospheric pressure plasma.
 8. The system of claim 1,wherein the plasma comprises a non-thermal plasma used for the removalof organic contaminants, coatings, adhesives and sealants.
 9. The systemof claim 1, wherein the second primary winding wraps around thesecondary winding.
 10. The system of claim 1, wherein the second primarywinding is offset axially from the secondary winding.
 11. The system ofclaim 1, wherein the second primary winding is displaceable from thesecondary winding to alter a coupling coefficient of the transformer.12. The system of claim 1, further comprising respective bobbins forholding the first primary winding, the secondary winding, and the secondprimary windings in place around the magnetic core.
 13. The system ofclaim 1, wherein the respective bobbins have holes for air cooling. 14.The system of claim 1, wherein annular gaps between bobbins supportingeither the first primary winding, the second primary winding or thesecondary winding provide cooling air.
 15. The system of claim 1,wherein at least one of the first primary winding and the secondarywinding is disposed offset from the magnetic core.
 16. The system ofclaim 11, wherein the first primary winding is offset axially from themagnetic core.
 17. The system of claim 1, wherein the alternatingcurrent (AC) power source comprises a square wave inverter.
 18. Thesystem of claim 1, further comprising a controller configured to atleast one of: control a gas flow through the plasma load, and control anoperational frequency of the AC power source.
 19. The system of claim18, wherein the controller is configured to: via the ballast transformerin claim 1 comprising an asymmetric ballast transformer having asufficient leakage inductance to prevent current surges, control the ACpower source such that a plasma is ignited and developed into a steadystate atmospheric pressure plasma.
 20. A method using any of the systemstatements above for providing power to a plasma, the method comprising:providing power from an AC power source to a plasma load via theasymmetric ballast transformer in any of the statements above having asufficient leakage inductance to prevent current surges; and ignitingand developing a steady state atmospheric pressure plasma.